1. Field of the Invention
The present invention generally relates to a diagnostic apparatus for detecting a failure in an evaporative fuel purge system, and more particularly to a diagnostic apparatus for an evaporative fuel purge system in which an evaporative fuel from a fuel tank is made to adhere to an absorbent in a canister in an internal combustion engine and fuel is fed to an intake passage of the engine.
2. Description of the Prior Art
Normally, internal combustion engines are equipped with evaporative fuel purge systems which prevent fuel vapor evaporated in a fuel tank from being emitted into the atmosphere. Structural elements of the fuel feeding system are hermetically sealed. The fuel vapor is made to adhere to an absorbent in a canister. Then, the fuel in the canister is sucked in an intake air passage of the internal combustion engine while the engine is operating. If a failure in the evaporative fuel purge system has occurred, fuel vapor may be emitted into the atmosphere. Hence, it is necessary to detect a failure in the evaporative fuel purge systems.
U.S. patent application Ser. No. 895,102 filed on Jun. 8, 1992, assigned to the common assignee of the present application, has proposed an apparatus for detecting a failure in the evaporative fuel purge system having first and second control valves. The first control valve, which is provided in a purge passage connecting the canister and the intake passage of the internal combustion ,engine, functions to connect the canister and the intake passage to each other and disconnect them from each other. In order to determine whether or not a failure in the evaporative fuel purge system has occurred, the second control valve is closed, and then the first control valve is closed after the system reaches a predetermined negative pressure. Then, the evaporative fuel control system is maintained in the closed state for a predetermined period, and variation in the pressure in the closed system is monitored during the period. Then, it is determined whether or nor a failure in the evaporative fuel purge system has occurred on the basis of the monitored variation.
Other types of evaporative fuel purge systems have been proposed in U.S. patent application Ser. No. 777,757 filed on Oct. 10, 1991, assigned to the common assignee of the present application.
However, conventional diagnostic apparatuses for the evaporative fuel purge systems do not consider anything which has an influence on the pressure in the purge system. Hence, an erroneous diagnosis decision may be made.